The reinforced composite industry has historically used reinforcing fibers, such as glass, in the form of continuous or chopped fibers, strands, and rovings to reinforce polymer matrices. These are used to make a wide range of composite products that possess a high degree of resilience and load-bearing ability. Such composite products may also be manufactured to possess decorative characteristics such as patterns, surface embossing, and coloration.
Glass reinforced polyolefin composites can be found in automotive, electrical and household appliance industries. Their use often requires combinations of specific mechanical, physical, chemical, and aesthetic properties. In many reinforced polyolefin composite applications, high strength, high resistance to chemical degradation and improved coloring are highly desirable properties. It is also highly desirable to produce polyolefin composites with mechanical properties such as low tensile creep and high resistance to fatigue. These parameters are considered when predicting the composite parts useful life span, and also when designing the composite part, often affecting its final thickness and weight.
The sizing composition plays a key role in determining the properties of the reinforced composite part. During manufacturing of the composite part, the fiber-size composition forms an interphase between the reinforcing fiber and the polymer matrix. When a load is applied to the composite part, force is transferred from the matrix to the fibers. A strong interphase is desired for high composite strength. High composite strength can be achieved with good adhesion of the fiber surface to the interphase, as well as from good adhesion between the interphase and the polymer matrix.
Good adhesion between the interphase and polymer matrix is generally achieved by the use of an appropriate fiber-size composition applied to the fibers. Although it may be relatively easy to tailor and improve a single specific property of the composite, it is difficult to improve several properties at the same time. For example, a sizing composition may be used to form a composite part with good initial strength. However, this composition may not form a composite with other properties such as good hydrolysis and detergent resistance, or good resistance to discoloration.
Therefore, it is desirable that the fiber-size composition form an interphase that is strong, resistant to thermal degradation, resistant to chemical degradation, provides good adhesion between the fiber and fiber-size composition, and provides good adhesion between the fiber-size composition and the polymer matrix. Also, the fiber-size composition must be compatible with both the reinforcing fibers, which may be inorganic, and the polymer matrix, which may be organic. Sodium, potassium, and calcium tetraborates and sodium borohydride are reported in Japanese Kokai 10[1998]291841 and 10[1998] 324544, respectively, as improving the performance of epoxy and urethane sizing, it being noted that epoxy resins have a poor adhesion to the reinforcing fiber while polyurethane, although having good adhesion to the reinforcing fiber, adheres poorly to the matrix resin.
In order to achieve composites with improved color, it is necessary to have a fiber-size composition comprising thermally stable ingredients that provide high resistance to oxidation and yellowing. As used here, the term “size” or “sizing” refers to a coating that is applied initially to forming filaments of a fiber for the purpose of protecting the fiber from abrasion breakage of the fibers during further processing of the fibers and subsequently promoting the adhesion between the fibers and the materials which they reinforce. While some physical binding between filaments may occur when the filaments are bundled into threads, it is essential that the sizing not interfere with the dispersion of the fibers in the matrix into which the fibers are incorporated. That is, the sizing should not have a tendency to agglomerate the threads, especially when incorporated into a matrix composition. This is in contrast to a “binder” where the formulation promotes the binding of threads to each other at their intersection (crossing points) in such forms as mats, fabrics and nonwovens through the polymerization of the binder while it is in contact with the fibers.
In binder applications, the major emphasis is on binding the threads together at their intersection in order to provide mat strength and stability. Unlike a size where application during filament formation is the norm and the size may be the only composition applied to the fiber prior to its final use, binders are typically used in addition to separate size compositions and are applied much later in the manufacturing process after the size has been applied. One of the purposes of a size is to coat the entire filament in order to protect the filaments and fibers during initial formation of the filaments and fibers and in their subsequent processing.
Binders are applied in a separate process after the filaments and fibers have been sized and processed to their final form and are used to bind and to hold firmly individual fibers to each other at their intersection or crossing points with each other. In a size, the emphasis is on bond formation between moieties already existing on components in the size composition and moieties found on the glass and between moieties found on components in the size composition and moieties found in the matrix resin typically with minimal polymerization of the components found in the size composition. A size typically solidifies on the fiber principally as a result of physical water removal whereas a binder is designed for a chemical (typically polymerization) reaction that gives a stronger fiber to fiber binding.
Traditionally, sizing compositions used in polypropylene composites are characterized by an aqueous emulsion of a film former having a highly modified polypropylene resin of low molecular weight. For example, ChemCorp 43N40, an aqueous emulsion of a maleic anhydride grafted polypropylene resin (E43 from Eastman Chemical Company) may be used as the main film forming agent in a sizing composition. E43 has an average molecular weight of 9000, and represents a resin with relatively low molecular weight. Although a sizing composition based on this film former is compatible with the reinforcing fibers and the polypropylene matrix resin, the final interphase formed is not strong due to the lower mechanical strength of this film former. Composite parts made from this sizing composition may possess insufficient short-term and long-term mechanical properties.
Additionally, in many similar sizing compositions, the surfactant package used in the film former emulsion contains low molecular weight chemicals which may be unsaturated, have one or more amine groups, or have amino groups which may be characterized as cationic in nature. These chemicals contribute to poor composite properties such as the discoloration of the composite part. Examples of these chemicals are unsaturated fatty acids (such as oleic, linoleic, and linolenic acids) and amine based neutralizing agents (such as triethylamine and nitrogen containing cationic surfactants). These agents can further cause yellowing and discoloration of the composite. Such properties make the final composite part unsuitable for many applications, and limit their use. Therefore, there is a need for a fiber-size composition that overcomes these problems.
Discoloration in molded composite products, or in the materials used to manufacture molded composite products, may arise from the presence of contaminants in one or more materials that make up the composite formulation, or from the presence of impurities in the ingredients that are used to form fiber-reinforced composites. These ingredients may be materials used in fiber-size compositions for coating reinforcing fibers before they are molded into composites. For example, conventional sizing compositions often impart a yellow color or other discoloration to fiber reinforcements after such sizings are applied. These discolorations are then carried over into the composite product when the reinforcements are molded. These discolorations may be caused by oxidative decomposition of unsaturated chemicals, such as fatty unsaturated surfactants and/or lubricants, which are of low thermal stability. These discolorations may also be caused by nitrogen containing compounds, such as amides, imides, cationic surfactants or amine-based chemicals, which are used, for example, as neutralizing agents.
Historically, the problem of discoloration has been partially addressed by adding ingredients to the composite formulation to counteract the discoloration before the composite formulation is molded. Frequently, antioxidants are used in the compounding formulations to minimize thermal degradation and associated discoloration. Also, the added ingredient may be a colorant, e.g., pigment or dye, that changes the color of the composite formulation. For example a blue pigment or dye may be added to the composite formulation to combat a yellow discoloration and, as a result, the finished molded composite appears whiter.
A more recently developed method of correcting discoloration has been adapted to fiber-reinforced composite manufacturing. Although, it has traditionally been used in compositions applied to paper products, clothing, and plastics to create a brilliant white appearance, this method involves adding an optical brightener, such as a fluorescent whitening or brightening agent, to the composite formulation or to the sizing compositions that are applied to the fiber reinforcements used to mold composites. U.S. Pat. No. 5,646,207, for example, describes a sizing composition that includes a fluorescent whitening agent in addition to other sizing ingredients such as a carboxylated polypropylene, a silane coupling agent, and a lubricant. However, compositions such as those disclosed in this patent rely specifically on the presence of the fluorescent whitening agent to reduce discoloration in the composite product. A related patent, U.S. Pat. No. 6,207,737 discloses the use of various stabilizers such as phosphinates, phosphonites, phosphties, hypophosphites, sulfites and bisulfites that are reported as effective in deterring oxidation of the matrix polymer in which the material is used. Preferably such stabilizers are used with a fluorescent whitening agent.
Use of an optical brightener does not, however, satisfactorily solve the problem of discoloration in the molded composite. According to U.S. Pat. No. 5,646,207, discoloration problems in the molded composite remain when the fluorescent whitening agent is added to the composite formulation because, in order to prevent discoloration satisfactorily, the fluorescent whitening agent must be well dispersed into the matrix polymer of the composite formulation. At the same time, the patent notes that uniform dispersion of the fluorescent brightener in the matrix polymer is difficult to achieve.
Other technical and economic problems stem from the use of optical brighteners such as a fluorescent whitening agent in composite formulations and in particular, in sizing compositions for fiber reinforcements. Technical problems may compromise the quality of the composite product, including degradation of the composite matrix polymer or undesirable interactions with other composite ingredients. For example, an optical brightener typically accelerates degradation of the matrix polymer when it is exposed to ultraviolet (UV) light or other forms of radiant energy. Moreover, optical brighteners themselves can degrade chemically over time, and thus contribute to yellowing or other discoloration of molded composite articles. Another observed problem arises when an optical brightener reacts with other ingredients such as an antioxidant that may be added to the composite formulation. In this regard, combining the optical brightener and the antioxidant reduces the efficiency of both ingredients, and ultimately results in discoloration of the composite.
Additionally, it has been observed that color matching of composite batches is difficult to achieve when the composite contains optical brighteners. In order to compensate for these difficulties in color matching, varying amounts of pigments or other additives have been added to the composite, which makes it difficult to maintain consistent color between batches. The difficulties encountered in turning out composite batches having consistent color, in turn, increase the cost of production by requiring more starting materials and higher labor costs, and therefore poses an economic disadvantage in addition to the technical problems. Further, color analysis of molded articles that contain optical brighteners is difficult because the articles behave differently under different lighting types and conditions. These problems with color analysis also increase the costs of producing the fiber reinforcements and/or the composite product. The use of optical brighteners further contributes to increased production costs simply because they are expensive chemicals.
In some applications, such as the manufacturing of washing machine parts, it may be desired that the molded composite product have a white color. In this regard, whitening pigments have been added directly to the composite molding composition to provide the white coloration. One such typically used whitening pigment is powdered titanium dioxide (TiO2). However, the addition of whitening pigments such as TiO2 results in damage to the reinforcing glass fibers and dramatically reduces the mechanical strength of the composite.
EP0826710 B1 and EP0826710 B1 disclose the use of a combination of tetrafluroborates and/or hypophosphinates as an accelerator in the curing of polyacids and bases to form a polymeric composition that is useful as a binder for binding nonwoven materials at the intersection or cross-over points of their individual fibers. Although useful in promoting the binder cross-linking reaction between polyacids with at least two carboxylic acid groups and hydroxyl or amine compounds, its use for a non-polymerization function such as found in sizing compositions is not mentioned or suggested. In U.S. Pat. No. 5,221,285, alkali metal dihydrogenphosphate, and alkali metal salts of phosphorous, hypophosphorous and polyphosphoric acids are used as catalysis in the esterfication and crosslinking of cellulose in textile form to polycarboxylic acids to form wrinkle resistant fabrics. Sodium borate and tetraborate, boric acid, and sodium borohydride are used to remove discoloration produced when cellulosic material is crosslinked with an alpha-hydroxy acid. Neither use is suggestive of use in a size composition.
Therefore, it is an object of the present invention to provide cost-effective fiber-size compositions.
It is an object of the present invention to provide increased whiteness to composite articles made with fibers sized with the fiber-size composition of the present invention.
It is an object of the present invention to provide increased brightness to composite articles made with fibers sized with the fiber-size composition of the present invention.
It is an object of the present invention to provide increased color compatibility to composite articles made with fibers sized with the fiber-size composition of the present invention.
It is an object of the present invention to provide increased whiteness, brightness, and/or color compatibility to composite articles made with fibers sized with the fiber-size composition of the present invention without requiring the use of an optical brightener.
It is an object of the present invention to provide increased whiteness, brightness, and/or color compatibility to composite articles made with fibers sized with the fiber-size composition of the present invention while maintaining desirable strength properties of the molded composite article.
It is yet another object of the present invention to provide composite articles made with fibers sized with the fiber-size composition of the present invention that are stable to oxidation degradation.
It is an object of the present invention to provide composite articles made with fibers sized with the fiber-size composition of the present invention that resist discoloration.
It is an object of the present invention to provide composite articles made with fibers sized with the fiber-size composition of the present invention that resist thermal degradation.
It is an object of the present invention to provide composite articles made with fibers sized with the fiber-size composition of the present invention that create a stronger interphase between the fiber and matrix resin.
It is an object of the present invention to provide composite articles made with fibers sized with the fiber-size composition of the present invention that have desirable short-term mechanical properties.
It is an object of the present invention to provide composite articles made with fibers sized with the fiber-size composition of the present invention that have desirable long-term mechanical properties.
It is another object of the present invention to provide composite articles made with fibers sized with the fiber-size composition of the present invention that have increased resistance to chemical breakdown.
It is an object of the present invention to provide composite articles made with fibers sized with the fiber-size composition of the present invention that have increased resistance to thermal deterioration.
The foregoing and other objects, features and advantages of the invention will become apparent from the following disclosure in which one or more preferred embodiments of the invention are described in detail. It is contemplated that variations in procedures may appear to a person skilled in the art without departing from the scope of or sacrificing any of the advantages of the invention.